Greases play an important role in technology, and are frequently used as lubricating substances. However, greases also find nonlubricating use, for instance, as cable filling compounds.
A grease typically is a solid or semiliquid substance comprising a thickening or gelling agent in a liquid carrier. The gelling agents used in greases frequently are fatty acid soaps, but high melting point materials such as clays, silica, organic dyes aromatic amides, and urea derivatives are also used. Nonsoap thickeners are typically present as relatively isometric colloidal particles. For instance, fumed silica particles typically are substantially spherical, and some clay particles tend to be plate-like. All types of gelling agents form a network structure in which the carrier is held by capillary forces.
When a low stress is applied to a sample of grease, the material acts substantially solid-like. If the stress is above a critical value, then the material flows and the viscosity decreases rapidly. The decrease in viscosity is largely reversible since it is typically caused by the rupture of network junctions between the filler particles, and these junctions can reform following the release of the supercritical stress.
A desirable property of a grease is the absence of syneresis, i.e., its ability to retain uniform consistency. Generally, syneresis is controlled by assuring dispersion of an adequate amount of colloidal particles or other gelling agent. Other desirable properties of grease compositions, such as oxidation resistance, are achieved or enhanced by means of appropriate additives.
The prior art knows greases filled with colloidal silica and other colloidal particles. For instance, U.S. Pat. No. 3,714,041 discloses organic liquids thickened with pyrogenic silica, silica aerogels, an fibrous asbestos. U.S. Pat. No. 4,396,514 discloses a lubricating composition comprising fumed silica. U.S. Pat. No. 4,265,775 discloses a nonbleeding thixotropic thermally conductive material comprising a silicone liquid carrier, silica fibers, and a thermally conductive filler powder.
Grease compositions have been used as cable filling material. In conventional (electrical) communications cables, filling compounds primarily serve to prevent water ingress into, and water propagation inside, the cable. In optical fiber cables, a further important function of a filling compound is the maintenance of the optical fibers in a low stress state.
Among known cable filling compounds are oil-extended rubbers and petrolatum. For an example of the former, see, U.S. Pat. No. 4,464,013, incorporated herein by reference, and of the latter, U.S. Pat. No. 4,333,706. The latter patent discloses a filling compound comprising a petrolatum base material and inorganic microspheres.
A cable filling compound, especially an optical fiber cable filling compound, should meet a variety of requirements. Among them is the requirement that the physical properties of the cable remain within acceptable limits over a rather wide temperature range, e.g., from about -40 to about 76.degree. C. It is also desirable that the compound be substantially free of syneresis over the temperature range. Filling compounds for use in optical fiber cables also should have a relatively low shear modulus. According to the prior art, the shear modulus is a critical material parameter of optical fiber cable filling compounds since it is believed to be directly related to the amount of microbending loss of fiber in filled cable. For a discussion of microbending loss, see Optical Fiber Telecommunications S. E. Miller et al., Academic Press, N.Y. (1979), pp. 158-161.
Typically, microbending loss is more difficult to control at long wavelengths than at short ones. Thus the requirements on the mechanical properties of a fiber cable filling compound are typically substantially more severe for cable that is to be used at, e.g., 1.55 .mu.m than they are if the cable is to be used at shorter operating wavelengths, e.g., at 1.3 .mu.m. Although we have found that some prior art filling compounds perform quite satisfactorily at wavelengths up to about 1.3 .mu.m, we have also found that this is often not the case at longer wavelengths.
Because silica-based optical fibers typically have their lowest losses at or near 1.55 .mu.m wavelength, there is great interest in eventually operating optical fiber telecommunication systems at approximately that wavelength. Thus, it is important to have available the means for producing optical fiber cable that has no significant cabling-induced losses at long wavelengths, including at about 1.55 .mu.m. We are disclosing herein a novel grease composition that has properties which make it useful as, inter alia, an optical fiber cable filling compound for cable operating at about 1.55 .mu.m.